Abstract (No change from original, funded submission) The prevalence of e-cigarette use has increased dramatically in recent years, particularly among young adults. Because e-cigarettes deliver fewer toxicants than combustible cigarettes, they likely offer improved health outcomes for current smokers who switch completely. However, many smokers who try using e-cigarettes do not switch completely, abandoning e-cigarettes altogether or continuing to use both products. Thus, it is important to understand the factors that lead to these variable tobacco use patterns. One of the primary determinants of e-cigarette uptake and changes in smoking behavior is the reinforcement value delivered by e- cigarette products. Product characteristics that impact nicotine delivery and sensorimotor characteristics are most likely to impact relative reinforcement value. Two of the primary product-level determinants of nicotine delivery and sensorimotor characteristics are the nicotine concentration of e-liquid and the power of the e- cigarette device. The proposed research project will investigate the impact of these characteristics on relative reinforcement value and tobacco use patterns. Current smokers (n=180) will receive an e-cigarette and e-liquid to take home and use over a three-week period. In a double-blind 2x2 design, participants will be randomly assigned to receive a low (3 mg) or high (12 mg) nicotine concentration, and either a low (20 W) or high (50 W) e-cigarette power setting. Participants will return to the lab each week to complete laboratory assessments of relative reinforcement value and provide breath and urine samples for biomarker assessments of smoke and nicotine exposure. Participants will also complete daily electronic diaries assessing tobacco use. Thus, the proposed design includes both lab-based and ecological assessments of reinforcement and tobacco use. Equal numbers of young adult (18-25) and older adult (>25) participants will be recruited to allow for exploratory analyses on the impact of age. The K01 candidate, Dr. Tracy Smith, aims to develop an independent research portfolio that seeks to understand how 1) contextual, individual, and product characteristics contribute to the use of tobacco products, and 2) we can leverage this information to improve public health through tobacco regulation, policy, and public health interventions. The proposed research project described here, along with clear training objectives, will provide the skills necessary to achieve that goal. Training objectives include gaining expertise in non-cigarette tobacco products, tobacco control research, developmental differences in tobacco use, applied research methodologies, and grant-writing. A strong team of mentors have been assembled with expertise in each of these areas. MUSC provides a stellar training environment to allow K-awardees to transition to independent research funding. The training described here is a clear progression from Dr. Smith's prior work, provides necessary skills for obtaining research independence, and will launch Dr. Smith's research independence in substantial, measurable ways.